OH! What a wonderfully helpful thread! I have one 
that actually took forever for me to figure out 
on my own without asking:

nli = not logged in (and now i use it alllllllll 
the time)

other forum acronyms:

FS = for sale

FA = for auction

FO = for offers

anime ones:

FY = fushigi yuugi

AnC = ayashi no ceres

RK = rurouni kenshin

MTT = Mahou Tsukai TAI (magic users club)

MKR = Magic Knights Rayearth

CB = cowboy bebop

NW = night walker


that's all i can think of now, some that I know 
have been covered already :D

~Tara